In the field of the present invention, micro electro-mechanical systems (MEMs) are included being the technology of mechanical components on the micrometer size, which includes three dimensional lithographic features of various geometries. They are typically manufactured using planar processing similar to semiconductor processes such as surface micromachining and/or bulk micromachining. MEMS are often fabricated using modified silicon fabrication technology, molding and plating, electro-discharge machining and other technologies capable of manufacturing very small devices.
The field of the present invention also embraces techniques that use nanometer-sized tips for imaging and investigating the structure of materials down to the atomic scale. Such techniques include scanning tunneling microscopy (STM) and atomic force microscopy (AFM), as disclosed in U.S. Pat. No. 4,343,993 and EP 0 223 918 B1.
Based on the developments in scanning tunneling microscopy and atomic force microscopy, new storage concepts have been introduced over the past few years that profit from these technologies. Probes having a nanoscale tip have been introduced for modifying the topography and for scanning an appropriate storage medium. Data are written as sequences of bits represented by topographical marks, such as indentation marks and non-indentation marks. The tips comprise apexes with a nanometer-sized diameter and the indentation marks have a comparable diameter, for example, a diameter in the range of 30 to 40 nm. Hence, these data storage concepts promise ultra-high storage area density.
In STM, a nanometer-sized tip is scanned in close proximity to a surface. The voltage applied therebetween gives rise to a tunnel current that depends on the tip-surface separation. From a data-storage point of view, such a technique may be used to image or sense topographic changes on a flat medium that represent a stored information in logical “0”s and “1”s. In order to achieve a reasonably stable current, the tip-sample separation must be maintained extremely small and fairly constant. In STM, the surface to be scanned needs to be a conductive material.
In AFM, the tip rests on one end of a soft spring cantilever. When the tip is brought in close proximity to a surface, resultant forces therebetween cause bending of the spring cantilever and so may be sensed.
A storage device for storing data based on the AFM principle is disclosed in “The millipede—more than 1,000 tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, March 2000. The storage device has a read and write function based on a mechanical x-, y-scanning of a storage medium with an array of probes each having a tip. During operation, the probes scan an assigned field of the storage medium in parallel. In this way, high data rates may be achieved. The storage medium comprises a polymethyl-methacrylate (PMMA) layer. The nanometer-sized tips are moved across the surface of the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. For this purpose, the probes comprise cantilevers which carry the tips on their end sections. Bits are represented by indentation marks or non-indentation marks in the polymer layer. The cantilevers respond to these topographic changes in the surface while they are moved across it.
Indentation marks are formed on the polymer surface by thermomechanical recording. This is achieved by heating a respective probe with a current or voltage pulse during the contact mode in a way that the polymer layer is softened locally where the tip touches the polymer layer. The result is an indentation, for example, having a nanoscale diameter, being formed in the layer.
Reading is also accomplished by a thermomechanical concept. The heater cantilever is supplied with an amount of electrical energy, which causes the probe to heat up to a temperature that is not high enough to soften the polymer layer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage medium, especially a substrate on the storage medium, changes when the probe is moving in an indentation as the heat transport is in this case more efficient. As a consequence of this, the temperature of the cantilever decreases and hence, its resistance changes. This change of resistance is then measured and serves as the measuring signal. Reading and/or writing the marks is accomplished by moving the probes relative to the storage medium in lines within a track and moving to the next track when the end of the respective line has been reached.
It is a challenge to provide a device and method for sensing a position of a probe.